Steam generators utilizing a heated primary fluid to produce steam from a secondary fluid have been used in nuclear power plants for a number of years. In such a steam generator the pressure of the steam is a function of the log mean temperature difference (LMTD) of the two fluids. The LMTD equation for a countercurrent flow heat exchanger is: ##EQU1## where T.sub.1 ' and T.sub.1 " are the temperatures of the entering first and second fluids, respectively and T.sub.2 ' and T.sub.2 " are the temperatures of the exiting first and second fluids, respectively, from the heat exchanger. If the flow pattern in the exchanger is not completely countercurrent or cocurrent, as in a U-tube heat exchanger, it is necessary to apply a correction factor. This equation does not include such a correction factor and is merely provided as an example. By increasing the LMTD the pressure of the steam can be increased. However, the temperature of the primary fluid is a limiting factor in a nuclear power plant because it is normally set at a maximum allowable value. Therefore, preheater or economizer chambers incorporated within the steam generator housing are important in providing an increased LMTD without requiring an increased primary fluid temperature. U.S. Pat. No. 3,804,069 issued to Bennett and assigned to Westinghouse Electric Corp., the assignee of the present invention, provides an example of a steam generator which includes a preheater located within the generator housing to raise the temperature of the secondary fluid to the boiling temperature thereof. The efficiency of the steam generator is improved by the operation of the preheater in rapidly raising the temperature of the secondary fluid to nearly that of the primary fluid.
Prior art preheaters presently used in steam generators generally use a laterally directed cross-flow system of heat transfer to maximize heat transfer in a limited amount of space. FIG. 1, set forth in more detail hereinafter, illustrates a steam generator wherein feedwater enters the generator through a nozzle and is directed back and forth across the cold leg side of a lower tube bundle section. However, due to the relatively high velocity of incoming secondary fluid which flows perpendicular to the tubes within each cross-flow path, current preheaters have potential to cause these tubes to vibrate. The resulting vibration of the heat exchanger tubes in the cold leg side of the generator causes them to strike against the support plates that laterally secure them, which might cause the walls of these tubes to wear. Such tube wall wear can in turn cause some leakage of the primary fluid that flows through the heat exchanger tubes to mix with the non-radioactive water secondary fluid that is ultimately used to create non-radioactive steam, thereby contaminating it. Consequently, the tube vibration that can potentially be produced by the lateral, back and forth flow pattern associated with prior art crossflow preheaters is undesirable.
Other drawbacks associated with such prior art preheaters result from the channeling of cold secondary feedwater near the base of the steam generator in the vicinity of the tubesheet and lower part of the tube bundle. The channeling of such cold feedwater onto the tubesheet results in unwanted thermal stresses to the lower shell and the tubesheet. Additionally, if the colder feedwater contacts saturated steam in the tube bundle, it can quench the steam, causing instantaneous steam collapse or water hammer. Water hammer conditions not only introduce unwanted mechanical shocks to the generator, but also produce undesirable water level fluctuations in the generator. Finally, the space required by some prior art preheaters within the shell of the steam generator displaces some of the U-shaped heat exchanger tubes that could otherwise be present, which lowers the overall efficiency of the generator.
To alleviate these latter drawbacks, systems have been designed to reduce the thermal stresses experienced by the tubesheet by splitting the flow of the incoming colder secondary water so only a limited amount of water is allowed to come in contact with the tubesheet as set forth in U.S. Pat. No. 3,896,770 issued to Byerley et al. Unfortunately, such preheater arrangements still may produce tube vibration. A second method is to provide a buffer zone adjacent to the tubesheet to protect the tubesheet from thermal shock, as set forth in U.S. Pat. Nos. 3,942,481 and 3,916,843 issued to Bennett. However, the baffle plates of the steam generator restrict service access to the cold leg areas of the tube bundle and introduce new sites where tube degradation may take place.
Clearly, a heat exchanger assembly is needed which can be incorporated into the steam generator in nuclear power plant facilities that reduces the potential for tube vibration present in cross-flow preheater systems while also providing a means of protecting the tubesheet and tube bundle from thermal shock stresses caused by contact with cold secondary fluid that maximizes the space available for exchanger tubes in the tube bundle.